1. Field of the Invention
The present invention relates to a driving assistance system that has an actuator for steering the steered wheels of a vehicle by applying a steering torque thereto, to thereby assist the driving of the vehicle.
2. Description of Related Art
Driving assistance systems that assist in preventing a vehicle from running astray from designated travel path using an image or video of the road ahead of the vehicle captured by a camera or the like and detecting the travel path based on the image or video are known. Such driving assistance systems are referred to as lane keeping (assist) systems. There are two types of lane keeping (assist) systems. One type only provides a warning to the driver when the vehicle deviates from its travel path. The second type actively corrects the deviation by steering (or assisting the steering of) the steered wheels by means of an actuator when the vehicle deviates from its travel path. Japanese Patent Application Publication No. JP-A-2001-10518 an example of the second type of system is disclosed in.
The vehicle steering control system disclosed Japanese Patent Application Publication No. JP-A-2001-10518 calculates the curvature of the road ahead of the vehicle, or the offset (an offset distance) and the yaw angle of the vehicle based on an image or video of the area ahead of the vehicle. Then, based on the road curvature, the offset and the yaw angle, the steered wheels are steered appropriately by means of an actuator in such a manner as to prevent the vehicle from deviating from its travel path. The offset is also called a transverse deviation and represents the deviation of the vehicle with respect to the travel path in a transverse direction thereof. The offset can be calculated based on an appropriate reference such as the longitudinal axis of the vehicle and/or the centerline of the lane in which the vehicle is traveling. The yaw angle is also called a heading angle and represents the vehicle's traveling direction with respect to the travel path.
In the vehicle steering control system disclosed in Japanese Patent Application Publication No. JP-A-2001-10518, however, the same steering control is applied regardless of the speed of the vehicle because the controller of the system is designed without taking variations of vehicle characteristics depending on the vehicle speed into consideration. In fact, when a vehicle is running at a high speed, a driver tends to grab the steering wheel firmly. In this situation, if the steered wheels are steered by employing the above-described steering control, a reaction force resulting from the steering action would be delivered to the driver, causing the driver to feel uncomfortable even if the reaction force is very small.